There lies an apparent race-specific parasitogenesis between potato Phytophthora infestants and potato plants. Such a specific relationship between a host and a pathogen is typically determined by the combination of avirulent genes contained in the pathogen and true resistant genes contained in the host. Dynamic resistance response is induced in the host when it is experiencing infection with an incompatible race. Specifically, production of reactive oxygen species, death of hypersensitive cell, production of phytoalexins (for potatoes, rishitin), expression of PR (Pathogenesis-Related) protein, formation of papilla, lignification and other resistance responses are triggered in the infected tissue, thereby arresting further development of the pathogen (See references 15, 32, 44, 45, and 47). In contrast, these resistance responses fail in the process of infection with a compatible race, allowing pathogen penetration and resulting in lethal, systemic infectious disease of potato.
One of the most important and local resistance responses among the dynamic resistance responses referred to above is thought to be accumulation of phytoalexin. Phytoalexins are small molecule compounds with microbiocidal action induced to accumulate upon pathogen infection, and has been indicated to be a crucial element in successful infection (See references 12, 13, 21, 28, and 46). Phytoalexins for potato are sesquiterpenoid compounds that are synthesized in isoprenoid metabolic system (FIG. 1).
It is known that isoprenoid synthesis in potato is rapidly converted from that for sterol/glycoalkaloid synthesis to that for sesquiterpenoid phytoalexin synthesis upon treatment with elicitor or inoculation of any incompatible race. This phenomenon is thought to be regulated by squalene synthase and sesquiterpene cyclase that act coordinately in the rate-limiting step of isoprenoid synthesis system to diverge the pathway into sterol glycoalkaroid synthesis and isoprenoid phytoalexin synthesis, respectively (See reference 8). Sesquiterpene cyclase for potato is vetispiradiene synthase and designated as potato vetispiradiene synthase (PVS) (See Reference 53). It has been reported that the PVS activity in potato tuber is significantly increased by inoculating pathogen or treating with HWC, which is derived from potato pathogen (See Reference 54). In addition, it is known that such an elicitor treatment leads to activation of sesquiterpenoid synthesis pathway also in tobacco plants, resulting in production of capsidiol, a type of phytoalexin (See References 42 and 48). Recently, these phenomena have been elucidated at the level of gene expression. As a result of Northern analysis of RNA extract from potato tuber using cDNA of PVS and squalene synthase isolated from potato as probes, it was found that PVS mRNA was temporarily induced to accumulate in the area where compatible and incompatible races had been inoculated. On the other hand, it has been shown that accumulation of mRNA for squalene synthase induced in the area of wound is suppressed by inoculation with compatible and incompatible races (See Reference 53). However, this report is not consistent with the observation that only the inoculation with incompatible race leads to biosynthesis of phytoalexin to arrest development of pathogen (See Reference 40).
It is generally known that most of plant genes constitute multigene families, with each isogene having discrete organ specificity and metabolic action in response to stimuli. It has been reported that PVS genes in potato plants also constitute multigene families having PVS 1 through 4 members (See Reference 53). Details of their behavior in expression are yet to be known.